During the drilling and completion of oil and gas wells, it may be necessary to engage in ancillary operations, such as monitoring the operability of equipment used during the drilling process or evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation temperature, formation pressure, bubble point, formation pressure gradient, mobility, filtrate viscosity, spherical mobility, coupled compressibility porosity, skin damage (which is an indication of how the mud filtrate has changed the permeability near the wellbore), and anisotropy (which is the ratio of the vertical and horizontal permeabilities). These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable and how to optimize production.
Wireline formation testers (WFT) and drill stem testers (DST) have been commonly used to perform these tests. The basic DST tool consists of a packer or packers, valves, or ports that may be opened and closed from the surface, and two or more pressure-recording devices. The tool is lowered on a work string to the zone to be tested. The packer or packers are set, and drilling fluid is evacuated to isolate the zone from the drilling fluid column. The valves or ports are then opened to allow flow from the formation to the tool for testing while the recorders chart static pressures. A sampling chamber traps formation fluid at the end of the test. WFTs generally employ the same testing techniques but use a wireline to lower the formation tester into the borehole after the drill string has been retrieved from the borehole. The WFT typically uses packers also, although the packers are typically placed closer together, compared to DSTs, for more efficient formation testing. In some cases, packers are not even used. In those instances, the testing tool is brought into contact with the intersected formation and testing is done without zonal isolation.
WFTs may also include a probe assembly for engaging the borehole wall and acquiring formation fluid samples. The probe assembly may include an isolation pad to engage the borehole wall. The isolation pad seals against the formation and around a hollow probe, which places an internal cavity in fluid communication with the formation. This creates a fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from the borehole fluid.
In order to acquire a useful sample, the probe must stay isolated from the relative high pressure of the borehole fluid. Therefore, the integrity of the seal that is formed by the isolation pad is critical to the performance of the tool. If the borehole fluid is allowed to leak into the collected formation fluid, a non-representative sample will be obtained and the test will have to be repeated.
Examples of isolation pads and probes used in WFTs can be found in Halliburton's DT, SFTT, SFT4, and RDT tools. Isolation pads that are used with WFTs are typically rubber pads affixed to the end of the extending sample probe. The rubber is normally affixed to a metallic plate that provides support to the rubber as well as a connection to the probe. These rubber pads are often molded to fit within the specific diameter hole in which they will be operating.
With the use of WFTs and DSTs, the drill string with the drill bit must first be retracted from the borehole. Then, a separate work string containing the testing equipment, or, with WFTs, the wireline tool string, must be lowered into the well to conduct secondary operations. Interrupting the drilling process to perform formation testing can add significant amounts of time to a drilling program.
DSTs and WFTs may also cause tool sticking or formation damage. There may also be difficulties of running WFTs in highly deviated and extended reach wells. WFTs also do not have flowbores for the flow of drilling mud, nor are they designed to withstand drilling loads such as torque and weight on bit.
Further, the formation pressure measurement accuracy of drill stem tests and, especially, of wireline formation tests may be affected by mud filtrate invasion and mudcake buildup because significant amounts of time may have passed before a DST or WFT engages the formation after the borehole has been drilled. Mud filtrate invasion occurs when the drilling mud fluids displace formation fluid. Because the mud filtrate ingress into the formation begins at the borehole surface, it is most prevalent there and generally decreases further into the formation. When filtrate invasion occurs, it may become impossible to obtain a representative sample of formation fluid or, at a minimum, the duration of the sampling period must be increased to first remove the drilling fluid and then obtain a representative sample of formation fluid. Mudcake buildup occurs when any solid particles in the drilling fluid are plastered to the side of the wellbore by the circulating drilling mud during drilling. The prevalence of the mudcake at the borehole surface creates a “skin”. Thus there may be a “skin effect” because formation testers can only extend relatively short distances into the formation, thereby distorting the representative sample of formation fluid due to the filtrate. The mudcake also acts as a region of reduced permeability adjacent to the borehole. Thus, once the mudcake forms, the accuracy of reservoir pressure measurements decreases, affecting the calculations for permeability and producibility of the formation.
Another testing apparatus is the formation tester while drilling (FTWD) tool. Typical FTWD formation testing equipment is suitable for integration with a drill string during drilling operations. Various devices or systems are used for isolating a formation from the remainder of the borehole, drawing fluid from the formation, and measuring physical properties of the fluid and the formation. Fluid properties, among other items, may include fluid compressibility, flowline fluid compressibility, density, resistivity, composition, and bubble point. For example, the FTWD may use a probe similar to a WFT that extends to the formation and a small sample chamber to draw in formation fluid through the probe to test the formation pressure. To perform a test, the drill string is stopped from rotating and moving axially and the test procedure, similar to a WFT described above, is performed.